Is there a significant difference in the metabolic effect (or weight loss success or blood work improvement) between following the paleo eating plan at 80% versus following the paleo eating plan at 100%? I would suspect that zero-carb fans would argue yes. What I'm looking for is links to any academic research (if some has been done) or anecdotal evidence. I understand that theoretically there is a difference; but I'm wondering if the difference is statistically significant.
To clarify, I mean 20% to be one non-paleo meal (or day) per week that would likely include whole fermented grains, fruits and very little refined sugar. I'm not talking about eating a whole cake and fettucini alfredo. For example, should I allow myself a few "cattle" foods now and again or should I go completely hard core because if you're not 100% you're not actually paleo...?
Or let me ask a different question: if I'm going to "cheat" anyway, should I cheat big once a month, or just a little each day? Again, for example, what if I, once or twice a week, sneak a little teaspoon of rice or a bite of potato that I make for my non-paleo husband (when I'm having veggies and a broiled tuna steak), have I ruined my whole day?
Will a teaspoon of starches immediately cause me to switch back to burning sugar for fuel? Even though the rest of the day has been awesomely paleo?
I suspect that it has more to do with how you define 20%.
20% carbs in an otherwise fat-heavy paleo meal? The carbs will drive insulin production, and end up storing a lot of that meal as fat.
One non-paleo meal/day a week? Probably not as impactful. Plenty of people see health improvements from doing intermittent fasting without adjusting their macronutrient intake, and IF and Paleo low-carb have similar effects on insulin.
I also suspect it's easier to regulate an off-day than an off-percentage. I can see 20% creeping up pretty easily.
As far as I'm aware there are no biological or physical benefits to 'cheating' on your paleo diet. So physically 100% is best. The 80/20 principal, or 90/10 is for your mental health. For fitting into society, taking a break, treating yourself.
Perhaps the real question is: Can you handle being 100% paleo? In this day and age with all the marketing and inundation of carb'd options, traveling and hectic work schedules, can you feasibly be 100% paleo. If Mark Sisson has planted the 80/20 in your mind, realize that his point is not to plan to be 80% paleo. Rather that you should cut yourself some slack if life's inevitable hand keeps you from being able to eat paleo you won't punish yourself with guilt.
It depends on what is in that 20%. Back when I started my 20% was things like candy...and of course that made me feel crappy. Luckily, since I was eating mostly paleo, I was able to recognize that feeling crappy really was caused by junk.
These days my 20% is properly fermented non-gluten grains and legumes- dosas, traditional grits, miso, buckwheat pasta, etc. These are traditional foods of healthy agrarian societies. They aren't as good as grass fed beef or wild salmon, but I sincerely doubt that they cause real problems.
I feel that in going from 80% paleo to 100% you may face the "law of diminishing returns." The effort required to get to 100% paleo may yield less than a corresponding 20% health benefit. This is similar to what Kurt Harris describes in his PaNu 12 steps (see http://www.paleonu.com/get-started/ ). And of course the basis of the question comes from the "80/20 rule" of Mark Sisson. I've found that "bearing down" on my workouts or further refining my diet can sometimes get me over a plateau in my health or fitness goals, but I have a ways to go before I'm 100% paleo.
If you're talking about being 20% less paleo then yes, in abstraction you'll get 20% less of the benefits.
On the 20% more carb rather than, say, zero carb question, there are lots of arguments that cutting out that last bit of (non-paleo) carb could have positive effects. There are some arguments that there's a threshold effect, such that you need to lower carb a certain amount before you see any real benefits. (This is why a lot of scientific studies on "high fat" diets are rubbished- because they contain 20% sucrose!)
How much causes problems will vary. For most, a single cookie on a paleo diet won't make much difference, other people swear they stop losing weight if they so much as look at an apple. It will depend on your activity levels, whether your metabolism is broken etc.
Of course it goes without saying that although you frame this as 80:20 vs ZC, a lot of the downsides will come from the grains themselves, not the carb, making it impossible to quantify. If you're gluten intolerant then a surprisingly small amount of grain might be important.
One thing you may need to watch out for in that 20% is that you really still need to avoid gluten and legumes as much as possible. These have long lasting(weeks) negative effects on gut permeability. Antibodies to gluten have been shown to persist for weeks after exposures, and some say it can affect you for up to a year. Dairy, white rice, and corn seem to be less detrimental cheats, so you are going to go for that cheat meal, those are better choices than bread and other grain products.
Aim for the 100%, but don't obsess if you 'fail' (what a bad word-choice). If you notice that one particular fail/cheat causes a bad reaction, try to avoid it in the future.
I have repeatedly noticed that a slight amount of grains causes my psoriasis to get worse, so I really try to avoid it.
On the other hand, eating real dark chocolate in greater amounts doesn't give me the physical signs, but makes it harder to stay away from sweets and carbs in general the entire week afterwards. That's tough mentally ;-)
By my definition, I cheat very little. However, I do not consider wine, dark chocolate, potatoes, limited white rice, some corn, dairy nor traditionally prepared beans to be cheats. BTW, I don't call myself Paleo, Primal nor Archevore but I am closest to Primal.
I want very much for Paleo/Primal/Archevore to change conventional wisdom. That could have enormous benefits to society. IMHO, only a looser, 80/20 approach could ever take hold in society at large.
You're not going to find any academic research in this regard, if only because so little research has been done on paleo and none of it comparing controlled caloric intake of paleo vs. any other diet. Second, what does the other 20% consist of? If you're of the dogmatic paleo persuasion, you frown at legumes, dairy, and grains. However, the longest lived civilizations all include these in their diet so I'm not one to poo-poo complex/whole versions of these foods. That's a long way of saying: if you're 80% there, miring in minutia isn't going to (likely) make much of a difference.